1. Field of the Invention
The present invention relates to a ferrule product, a method of making the same, and an optical module.
2. Related Background Art
As the transmission capacity of optical communications increases, wavelength division multiplexing transmission systems (hereinafter referred to as WDM transmission systems) have come in to use. Inconventional WDM transmission systems, those having a channel interval as narrow as about 50 GHz (wavelength interval of 400 pm) have been in practical use. As the light source for such a WDM transmission system, DFB lasers have mainly been in use.
Further increases in the transmission capacity of optical communications have recently been in demand. For responding to such a demand, WDM transmission systems having a narrower channel interval are necessary. However, the oscillation wavelength of a DFS laser greatly depends on its operating temperature and injection current. Therefore, when a DFB laser is used as a light source, a deterioration in transmission characteristic may occur in adjacent channels due to optical crosstalk. This optical crosstalk occurs, for example, when the oscillation wavelength greatly shifts due to fluctuations in current generated by changes over time during a long-term continuous operation. Since it is necessary for high-density WDM systems to narrow their channel intervals in particular, the occurrence of optical crosstalk may become a serious problem therein. Hence, an external cavity type fiber Bragg grating semiconductor laser constituted by a fiber Bragg grating and a semiconductor optical amplifier has been developed as a light source with further wavelength stability.
As disclosed in a literature (literature 1: M. Ziari, A. Mathur, H. Jeon, I. Booth and R. J. Lang, xe2x80x9cFIBER-GRATING BASED DENSE WDM TRANSMITTERS,xe2x80x9d NFOEC97, pp. 503-512.), the oscillation wavelength of a fiber Bragg grating semiconductor laser is exclusively determined by the Bragg wavelength of the Bragg grating. Therefore, the temperature dependence of the oscillation wavelength of a fiber Bragg grating semiconductor laser is also determined by the temperature dependence of the Bragg wavelength of the fiber Bragg grating (which is very low, i.e., about 10 pm/K), and its dependence on the injection current to the semiconductor optical amplifier is also very small. Hence, its change in oscillation wavelength with respect to temperature and injection current is remarkably smaller than that in DFB lasers. In addition, when making a fiber Bragg grating, the Bragg wavelength of the fiber Bragg grating can be controlled so as to attain a desirable value more accurately than that of DFB lasers. Therefore, as disclosed in several literatures (literature 2: F. N. Timofeev, P. Bayvel, V. Mikhailov, O. A. Lavrova, R. Wayatt, R. Kashyap, M. Robertson, and J. E. Midwinter, xe2x80x9c2.5 Gbit/s directly-modulated fiber grating laser for WDM networks,xe2x80x9d Electron. Lett. 1997, pp. 1406-1407.; literature 3: M. Ziari, A. Mathur, H. Jeon, I. Booth and R. J. Lang, xe2x80x9cFIBER-GRATING BASED DENSE WDM TRANSMITTERS,xe2x80x9d NFOEC97, pp. 503-512.; literature 4: T. Takagi, T. Kato, G. Sasaki, A. Miki, S. Inano, K. Iwai, A. Hamanaka, M. Shigehara, xe2x80x9cFiber-grating external-cavity laser diode imodule for 2.5 Gb/s dense WDM transmission,xe2x80x9d Proc. ECOC 1998, pp.81-82.; and literature 5: T. Kato, T. Takagi, A. Hamanaka, K. Iwai, and G. Sasaki, xe2x80x9cFiber-Grating Semiconductor Laser Modules for Dense-WDM Systems,xe2x80x9d IEICE TRANS. ELECTRON., VOL. E82-C. No. 2, 1999, pp. 357-359.), fiber Bragg grating semiconductor lasers are remarkably superior to DFB lasers in terms of stability and controllability in wavelength.
As mentioned above, the oscillation wavelength of a fiber Bragg grating semiconductor laser is determined by the Bragg wavelength corresponding to the grating period of its fiber Bragg grating. Therefore, basically the temperature change rate of the oscillation wavelength of a fiber Bragg grating semiconductor laser should be nearly equal to the temperature change rate of the Bragg wavelength of its fiber Bragg grating. Namely, the temperature change rate of the Bragg wavelength of a fiber grating should be about 10 pm/K.
The inventors measured the temperature change rate of the oscillation wavelength of available fiber grating semiconductor lasers. In this measurement, the inventors found that the value of temperature change rate was about 17 pm/K, thus being remarkably greater than the value expected so far.
The inventors carried out various investigations and studies in order to solve the cause of sluch an abnormal value of temperature dependence. As a result, regarding the temperature dependence of oscillation wavelength, the inventors took notice of the structure of a ferrule for holding an optical fiber. FIG. 13 is a sectional view showing a ferrule product 8 used for technical studies. The ferrule product 8 comprises an optical fiber 81 and a ferrule 82. The ferrule 82 has a capillary part 821 for holding the optical fiber 81 and a flange part 822 for holding the capillary part 821.
As a result of various studies concerning the ferrule product 8, the inventors have found the tollowing fact: The material characteristics and structures of the flange part 822 and capillary part 821 affect the temperature change rate of the Bragg wavelength of the fiber Bragg grating, thereby increasing the temperature change rate of the oscillation wavelength of the fiber Bragg grating laser.
As can be understood from the foregoing explanation, the inventors have found that the oscillation wavelength of fiber Bragg grating semiconductor lasers fails to exhibit its expected stability with respect to temperature. It is unfavorable for the oscillation wavelength to have a large temperature dependence in the case of temperature controlled type fiber Bragg grating semiconductor lasers using a temperature controlling device such as Peltier device, since the burden on temperature control becomes heavier. On the other hand, uncooled fiber Bragg grating semiconductor lasers without temperature controller will be in great demand in access network system if their temperature dependence at present can be significantly reduced. In any case, it is essential that the temperature dependence of oscillation wavelength in fiber Bragg grating semiconductor lasers be lowered so as to achieve the required temperature characteristic.
Therefore, it is an object of the present invention to provide a ferrule product which can reduce the temperature change rate of the Bragg wavelength of fiber Bragg gratings, and a method of making the same; and further to provide an optical module which can reduce the temperature change rate of the oscillation wavelength of a fiber Bragg grating semiconductor laser.
One aspect of the present invention is the ferrule product utilized in an optical module comprising a semiconductor optical amplifier having a pair of end facets. The ferrule product comprises an optical fiber and a ferrule. The optical fiber has one end portion provided so as to be optically coupled with one of the pair of end facets of the semiconductor optical amplifier, a first optical fiber portion including a Bragg grating provided at a predetermined position distanced from the one end portion, and a second optical fiber portion different from the first optical fiber portion. The ferrule has a capillary portion for holding the optical fiber and a flange portion for holding the capillary portion, whereas the flange portion is provided on the second optical fiber portion.
According to this aspect of the present invention, since the flange portion is provided on the second optical fiber portion, the force applied by the flange portion onto the first optical fiber portion of the optical fiber as temperatire rises can be reduced. Therefore, the change in grating period with respect to temperature can be reduced.
In the ferrule product, material of the flange portion may have a coefficient of linear expansion smaller than that of material of the capillary portion. When the flange portion is deformed by heat, the thermal stress from the flange portion onto the capillary portion is reduced, whereby the stress from the capillary portion onto the optical fiber is also alleviated.
In the ferrule product, a material of the capillary portion may have a negative coefficient of linear expansion, whereas a material of the optical fiber may have a positive coefficient of linear expansion. The capillary portion having a negative coefficient of linear expansion shrinks as temperature rises, whereas the optical fiber having a positive coefficient of linear expansion expands as temperature rises. As a consequence, the optical fiber and the capillary portion reduce deformations therebetween. Therefore, the change in grating period with respect to temperature is reduced.
The ferrule product may be configured such that a material of the capillary portion has a positive coefficient of linear expansion smaller than that of zirconia, whereas the optical fiber has a positive coefficient of linear expansion. In this case, since the capillary portion is formed from a material having a positive coefficient of linear expansion smaller than that of zirconia, the magnitude of thermal stress applied by the capillary part onto the fiber becomes smaller than that in the case where zirconia is used, although the direction of thermal stress is the same. As a result, the change in grating period with respect to temperature is reduced. A preferred example of the material for forming the capillary portion is crystallized glass.
The ferrule product may further comprise a bonding member for bonding the capillary portion and the flange portion to each other. The capillary portion may contain at least one material of crystallized glass and negative thermal expansion glass ceramics. The bonding member makes it possible for the flange portion to hold the glass capillary portion exhibiting a low toughness.
Another aspect of the present invention is the ferrule product utilized in an optical module comprising a semiconductor optical amplifier having a pair of end facets. The ferrule product comprises an optical fiber and a ferrule. The optical fiber has one end portion provided so as to be optically coupled with one of the pair of end facets of the semiconductor optical amplifier, and a Bragg grating provided at a predetermined position distanced from the one end portion. The ferrule has a capillary portion for holding the optical fiber and a flange portion for holding the capillary portion. Material of the capillary portion has a negative coefficient of linear expansion, whereas material of the flange portion has a positive coefficient of linear expansion.
According to this aspect of the present invention, since the capillary portion is formed from material having a negative coefficient of linear expansion, the capillary portion shrinks as temperature rises, since the flange portion is formed from material having a positive coefficient of linear expansion, the flange portion expands as temperature rises. As a consequence, the capillary portion and the flange portion reduce deformations therebetween. Since the deformations partly cancel each other, this cancellation can reduce the deformation of the optical fiber caused by the deformation of flange portion. Therefore, the change in grating period with respect to temperature can be reduced.
In the ferrule product, the capillary portion may contain a negative thermal expansion glass ceramics material having a longitudinal elastic coefficient exceeding 27 GPa. A material having a longitudinal elastic coefficient exceeding 27 GPa provides an excellent temperature dependence in Bragg wavelength. A further excellent temperature dependence in Bragg wavelength is provided when the longitudinal elastic coefficient in the capillary portion is 53 GPa or greater.
The ferrule product may further comprise a bonding member for bonding the capillary portion and the flange portion to each other. The bonding member makes it possible for the flange portion to reliably hold the glass capillary part exhibiting a low toughness.
Another aspect of the present invention is the ferrule product utilized in an optical module comprising a semiconductor optical amplifier having a pair of end facets. The ferrule product comprises an optical fiber and a ferrule. The optical fiber has one end portion and a Bragg grating. The one end portion is provided so as to optically couple with one of the pair of end facets of the semiconductor optical amplifier. The grating is provided at a position separated from the one end portion by a predetermined distance. The ferrule has a capillary portion for holding the optical fiber and a flange portion for holding the capillary portion. Material of the flange portion has a coefficient of linear expansion smaller than that of stainless steel.
According to this aspect of the present invention, since the flange portion is formed from material having a coefficient of linear expansion smaller than that of stainless steel, the force applied by the flange portion onto the capillary portion as temperature rises can be reduced. Therefore, the stress occurring in the capillary portion is reduced. As a result, the force exerted from the capillary portion onto the optical fiber is reduced. Hence, the change in grating period with respect to temperature is lowered. A preferred example of the material for forming the flange portion is covar.
An optical module can be constructed by using the ferrule product in accordance with the present invention. The optical module comprises a semiconductor optical amplifier having a pair of end facets, and a ferrule product. The ferrule product comprises an optical fiber and a ferrule. Since the change in grating period with respect to temperature is reduced in the ferrule product in accordance with the present invention, the temperature change rate of the oscillation wavelength of the optical module decreases.
In the ferrule product, a material of the flange portion may have a coefficient of linear expansion lower than that of material of the capillary portion. When the flange portion is deformed by heat, the thermal stress from the flange portion onto the optical fiber is reduced.
The capillary portion may contain at least one material of crystallized glass and negative thermal expansion glass ceramics. Each of these materials have a coefficient of linear expansion sufficiently smaller than that of zirconia.
The ferrule product may further comprise a bonding member for bonding the capillary portion and the flange portion to each other. The bonding member makes it possible to reliably assemble the glass capillary portion exhibiting a low toughness into the flange portion.
The optical module in accordance with the present invention may further comprise a mounting member, a tubular member, and a ferrule holder which are arranged along a predetermined axis. The mounting member has an arrangement face extending along a plane intersecting the predetermined axis, and a supporting portion, provided on the arrangement face, having a mounting face extending along the predetermined axis. The semiconductor optical amplifier is disposed on the mounting face. The tubular member extends along the predetermined axis, and has a first end portion secured to the arrangement face of the mounting member and a second end portion secured to the ferrule holder. The ferrule holder holds the ferrule along the predetermined axis. The optical fiber is provided as so to extend along the predetermined axis. Since the semiconductor optical amplifier is mounted on the mounting face, it is mounted without the aid of a temperature controlling device.
Another aspect of the present invention relates to a method of making a ferrule product. The ferrule product is utilized in an optical module comprising a semiconductor optical amplifier. This method comprises the steps of preparing a flange member having a first length, a capillary member having a second length longer than the first length, and an optical fiber comprising a first end portion and a Bragg grating, the Bragg grating being provided at a predetermined position separated from the first end portion, said Bragg grating having a third length, the capillary member comprising a first portion having at least the third length and a second portion having at least the first length; providing the flange portion at the second portion; and inserting the optical fiber into the capillary member such that the grating is positioned at the first portion.
This method makes it possible to insert the optical fiber such that the grating is positioned at the first portion of the capillary member, while the flange portion is arranged on the second portion of the capillary member.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.